1. Field of the Invention
The present invention relates to compact apparatus for generating intense X-rays suitable for lithography and other uses.
2. Description of the Prior Art
The continuing improvement in the performance of integrated circuits has depended to a large extent on an ability to produce progressively finer features on the surface of a silicon wafer. Optical lithography has provided the main production tool for reproducing these fine features. It has been developed to a remarkable level of sophistication and can now produce line widths as narrow as 0.7 .mu.m. Future developments in optical lithography are anticipated to reduce these line widths still further, but it is likely that progress below 0.5 .mu.m will raise severe processing difficulties; progress below 0.3 .mu.m will probably be impossible. X-ray lithography, on the other hand, offers the clear potential for resolving features as small as 0.1 .mu.m. More importantly, at much larger feature sizes of 0.5 .mu.m, it offers significant processing advantages over optical lithography. The most important of these are:
(a) large depth of focus (.apprxeq. 40 .mu.m);
(b) broad exposure latitude;
(c) broad processing latitude; and
(d) relative insensitivity to dust particles.
The generation of X-rays for use in X-ray lithography is typically provided by apparatus which utilizes synchrotron radiation rings.
Synchrotron radiation output power is produced by bending a beam of electrons in a magnetic field. This emitted power is focussed in one plane so that a wafer stepper may be located at a comfortable distance from the source. In comparison with alternative sources, the conventional synchrotron produces an illumination at the wafer stepper about 500 times greater than a rotating anode X-ray source and at least 20 times greater than a laser plasma or gas puff plasma source. This high level of illumination brings obvious benefits for increasing the throughput in a manufacturing environment. Good mask lifetime is assured by the fact that, unlike laser or plasma source, synchrotrons produce no debris.
In most synchrotron X-ray sources, the electron beam is produced in a separate electron linear accelerator (the injector). The conventional accelerator produces a gradient on the order of 15-20 MeV/meter. A 200 MeV injector is thus about 10-14 meters long. Electrons from the injector are injected into the synchrotron ring where they are made to circulate in a closed orbit by a suitable arrangement of bending and focussing magnets. The electrons may then be accelerated to higher energy by feeding rf power to an accelerating cavity while simultaneously increasing the field in the magnets. At full energy, the magnets remain at fixed field and acceleration ceases, but rf power must still be fed to the cavity in order to make good the energy loss sustained by the electron beam in emitting X-rays. In this "stored beam" mode, the electron beam may continue to circulate for some time period, emitting a steady beam of X-rays. Eventually the electrons start to be lost via scattering by residual gas molecules in the vacuum chamber and it is necessary to abort the remaining beam and re-start the injection process.
Electron storage rings using conventional magnets are to be found in all major industrialized countries and have now been accepted sources of X-ray radiation for research in many areas of science. Unfortunately, such installations are too large for use in a microchip fabrication facility because the relatively low fields available from conventional bending magnets imply large bending radii for the stored electron beam and therefore a large perimeter for the closed orbit ring. The more powerful fields available from superconducting magnets can be used to produce a much more compact installation. An example of a commercially available superconducting synchrotron that produces a steady X-ray power output is the Helios model, currently under development by Oxford Industries, Oxford, England. Using superconducting magnets, the Helios model bends the electron beam around a radius of only 0.5 meters. To produce the same X-ray wavelengths using conventional magnets would require a bend radius of 3 meters. A superconducting ring can accommodate electrons injected at one third the energy of a conventional ring thus reducing the injector cost. Efficiency is also gained by reducing the overall size of the installation and the required thickness of shielding, the latter resulting from the reduced overall size of the superconducting synchrotron.
Although the use of the Helios type synchrotron ring significantly reduces the overall size of the installation, the size of the electron linear accelerator used as the injector is still very large (typically 10-14 meters). This severely limits the use of the apparatus in commercial microchip fabrication facilities where space availability is at a premium.